The Solid State Lighting (SSL) Industry's demand for small and compact power management units for LEDs is increasing. This demand is even stronger in the field of Power LEDs, for which energy from the power supply has to be delivered in the form of a constant current as efficiently as possible.
“Driver on board” technology is known where the SSL light source and the driver are to be combined on one carrier board. Examples of this may be (i) SSL light sources being LEDs (with or without light conversion, such as direct, vicinity, remote or any other phosphor), combined with at least part of the driver on a printed circuit board; (ii) SSL light sources being LEDs, combined with at least part of the driver on another carrier, such as a silicon wafer, glass plate, (thermally enhanced) plastic carrier; (iii) SSL light sources being OLEDs, combined with at least part of the driver on the carrier of the OLED; (iv) Combination of laser and driver; and (v) Combination of light source, and other signal or power electronics.
In these “driver on board” scenarios, size is one constraint, either as a limitation in the footprint (the “real estate” of the driver circuit occupied on the board) and/or the height of the components. These limitations may originate from optical constraints, for example high components may block part of the light emission from a light source, or large surface components may impact the total reflectivity of a light mixing chamber.
To one end small size of the driver circuit is a target, however small size comes with low volume of components. To the other end high performance is the target, however low volume of components results in relatively limited performance, which challenges driver circuit design.
Multimode driver circuits are typically implemented by using a power converter. Multimode driver power converters allow regulating a constant output voltage with a plurality of ranges of conversion ratios, in order to cope with the variations of input voltage. The input supply voltage may be from either an Alternating Current (AC) input (from power grid) or a Direct Current (DC) input (battery, AC rectified voltage etc.). Voltage variations from these sources are unavoidable. For example, when power is coming from AC power grid, intrinsic quality of electrical generator and transmission loss through the wire will typically result in a 10% error providing the AC supply voltage to users. Adapters are used to rectify AC voltage to DC voltage and are controlled by electronic circuits. The performance (DC regulation ability, process variation, etc.) of these electronic circuits will provide different DC voltages. For a portable device powered by a lithium battery, the voltage from the battery will typically vary from 4.2V (fully charged) to 2.7V (fully used). The power conversion ratio is essentially proportional to the duty cycle in each operating range, wherein desired conversion ratio can typically be achieved through adjusting the duty cycle of a Pulse Width Modulation (PWM) signal.